No abstract

The structures and l3C NMR chemical shifts of carbonic acid and its mono-and diprotonated forms were calculated using ab initio and IGLO methods, respectively. The results were compared with the experimentally obtained NMR data under superacidic conditions.

Ab initio calculations at the MP2/cc-pVTZ level show that the cyclobutylmethyl cation is a nonclassical sigma-delocalized species, which is distinct from the global minimum C2-symmetric cyclopentyl cation (Schleyer, P. v. R.; Carneiro, J. W. de M.; Koch, W.; Raghavachari, K. J. Am. Chem. Soc. 1989, 111, 5475). Relatively lower level DFT calculations, on the other hand, show that the primary cyclobutylmethyl cation spontaneously collapses into the cyclopentyl cation (Prakash, G. K. S.; Reddy, V. P.; Rasul, G.; Casanova, J.; Olah, G. A. J. Am. Chem. Soc. 1998, 120, 13362). The secondary 1-cyclobutylethyl cation is also a nonclassical carbocation, as shown by calculations at the MP2/cc-pVTZ level. Two structures having energy minima are identified for the latter cation on the potential energy surface. The conformer in which the methyl group is in the exo orientation is a global minimum and is favored over the corresponding endo conformer by 1.2 kcal/mol at the MP2/cc-pVTZ//MP2/cc-pVTZ +ZPE level of calculations. The tertiary 1-cyclobutyl-1-methylethyl cation, at this level of calculations, also involves substantial nonclassical sigma-delocalization, showing that the nonclassical stabilization is more important for cyclobutylmethyl cations relative to the cyclopropylmethyl cations. The 13C NMR chemical shifts obtained from GIAO-CCSD(T)/tzp/dz calculations further substantiate the nonclassical structures for these carbocations.

Primary and secondary cyclobutylmethyl cations were found to be elusive in superacidic media even at low temperatures. They give thermodynamically more stable rearrangement products, i.e., the dimeric bicyclo[4.4.0]dec-1-yl cation and substituted cyclopentyl cations, respectively. In the search for a persistent cyclobutylmethyl cation, the more stabilized cyclobutyldicyclopropylmethyl cation was prepared from its corresponding alcohol in FSO3H/SO2ClF at −90 °C. Using variable-temperature 13C NMR studies and theoretical calculations, the ion was shown to exist preferentially in its bisected conformation at −80 °C. The ion is predominantly classical, with expected significant delocalization into the neighboring cycloalkyl rings.